Preheat oven to 350 degrees.
Take the pasta or rice, the meat and the veggies and dump them into a 9x9 baking dish.
Make the sauce by melting the butter in a medium saucepan over low heat. If you want to saute the onions or garlic or whatever add-in you’re using, use one more tablespoon of butter. Add the flour and stir until completely mixed into a thick paste. Add the milk and slowly bring to a boil. Lower heat and cook just until thick. Remove sauce from the heat. Add cheese if making a cheese sauce. Stir until melted. Add desired seasonings to taste. Pour over top of casserole mixture and stir gently until mixed well. Bake, uncovered, for 30 minutes. Let stand about 10 minutes before serving.